Separator for nonaqueous secondary battery, and nonaqueous secondary battery

ABSTRACT

An object of the invention is to provide a separator for a nonaqueous secondary battery, which has good adhesion to electrodes, is capable of ensuring sufficient ion permeability even after attachment to electrodes, and further includes an adhesive porous layer having dynamic physical properties sufficient to withstand heat pressing and a uniform porous structure. The separator for a nonaqueous secondary battery of the invention includes a porous substrate and an adhesive porous layer that is formed on at least one side of the porous substrate and contains a polyvinylidene-fluoride-based resin. The separator for a nonaqueous secondary battery is characterized in that the adhesive porous layer has a porosity of 30 to 60% and an average pore size of 1 to 100 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2011/074258 filed Oct. 21, 2011, the contents of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a separator for a nonaqueous secondary battery and also to a nonaqueous secondary battery.

BACKGROUND ART

Nonaqueous secondary batteries, such as lithium ion secondary batteries, have been widely used as power supplies for portable electronic devices such as laptop computers, mobile phones, digital cameras, and camcorders. Further, these batteries are characterized by having high energy density, and thus the application to automobiles and the like has also been studied in recent years.

With the reduction in size and weight of portable electronic devices, the outer casing of a nonaqueous secondary battery has been simplified. At first, a battery can made of stainless steel was used as an outer casing, and then an outer casing made of an aluminum can was developed. Further, a soft pack outer casing made of an aluminum laminate pack has also been developed nowadays. In the case of a soft pack outer casing made of an aluminum laminate, because such an outer casing is soft, a space may be formed between an electrode and a separator during charging and discharging, causing a technical problem in that the cycle life is reduced. In terms of solving this problem, a technique for attaching an electrode and a separator together is important, and a large number of technical proposals have been made.

As one of the proposals, a technique of using a separator including a polyolefin microporous membrane, which is a conventional separator, and a porous layer made of a polyvinylidene-fluoride-based resin (hereinafter sometimes referred to as an adhesive porous layer) formed thereon is known (see, e.g., Patent Document 1). When such an adhesive porous layer with an electrolyte contained therein is stacked on an electrode and heat-pressed, the electrode and the separator can be well joined together, allowing the adhesive porous layer to function as an adhesive. As a result, it is possible to improve the cycle life of a soft pack battery.

In addition, in the case where a battery is produced using a conventional metal can outer casing, electrodes and a separator are stacked together and wound to produce a battery element, and the element is enclosed in a metal can outer casing together with an electrolyte, thereby producing a battery. Meanwhile, in the case where a soft pack battery is produced using a separator like the separator of Patent Document 1 mentioned above, a battery element is produced in the same manner as for the battery having a metal can outer casing mentioned above, then enclosed in a soft pack outer casing together with an electrolyte, and finally subjected to a heat pressing process, thereby producing a battery. Thus, in the case where a separator including an adhesive porous layer as mentioned above is used, it is possible to produce a battery element in the same manner as for the battery having a metal can outer casing mentioned above. This is advantageous in that there is no need to greatly change the production process for conventional batteries having a metal can outer casing.

Against the background mentioned above, various technical proposals have been made in the past for separators made of a polyolefin microporous membrane and an adhesive porous layer stacked thereon. For example, in terms of achieving both the ensuring of sufficient adhesion and ion permeability, Patent Document 1 presents a new technical proposal focusing on the porous structure and thickness of a polyvinylidene-fluoride-based resin layer.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent No. 4127989

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

Incidentally, the positive electrode or negative electrode of an ordinary nonaqueous secondary battery includes a current collector and an active material layer that is formed on the current collector and contains an electrode active material and a binder resin. In the case where the adhesive porous layer mentioned above is joined to such an electrode by heat pressing, the layer adheres to the binder resin in the electrode. Therefore, in order to ensure better adhesion, the larger the amount of binder resin in the electrode, the better.

However, in order to further increase the energy density of a battery, it is necessary to increase the content of the active material in the electrode, and the lower the binder resin content, the better. Therefore, according to the prior art, in order to ensure sufficient adhesion, it has been necessary to perform heat pressing under severe conditions, such as higher temperatures and higher pressures. Further, according to the prior art, in the case where heat pressing is performed under such severe conditions, there is a problem in that the porous structure of the adhesive porous layer made of a polyvinylidene-fluoride-based resin is destroyed. This results in insufficient ion permeability after the heat pressing process, and it thus has been difficult to obtain good battery characteristics.

In addition, although a polyvinylidene-fluoride-based resin has been commonly used as a binder resin for use in an electrode, in recent years, it is also becoming common to use styrene-butadiene rubber. In the case where such styrene-butadiene rubber is used in an electrode, when the separator has a conventional adhesive porous layer, it has been difficult to obtain sufficient battery characteristics achieving both ion permeability and adhesion.

For example, in the separator described in Patent Document 1, the porous layer made of a polyvinylidene-fluoride-based resin has an extremely high porosity of 50 to 90%. However, such an adhesive porous layer having high porosity has a problem in that when subjected to a heat pressing process under severe attachment conditions, its porous structure is destroyed due to insufficient dynamic physical properties. Further, the surface of the adhesive porous layer is interspersed with pores having a pore size of 0.05 to 10 μm. However, when the surface has such a non-uniform pore structure, in the case where the amount of binder resin in an electrode is reduced and the heat-pressing conditions are relaxed, it is difficult to achieve adhesion to electrodes, ion permeability, and the cycle characteristics of a battery together.

Against such a background, an object of the invention is to provide a separator for a nonaqueous secondary battery, which has better adhesion to electrodes compared with the prior art, is capable of ensuring sufficient ion permeability even after attachment to an electrode, and further includes an adhesive porous layer having dynamic physical properties sufficient to withstand heat pressing and a uniform porous structure.

Means for Solving the Problems

In order to solve the problems mentioned above, the invention is configured as follows.

1. A separator for a nonaqueous secondary battery, including a porous substrate and an adhesive porous layer that is formed on at least one side of the porous substrate and contains a polyvinylidene-fluoride-based resin,

the separator for a nonaqueous secondary battery being characterized in that the adhesive porous layer has a porosity of 30% or more and 60% or less and an average pore size of 1 nm or more and 100 nm or less.

2. The separator for a nonaqueous secondary battery according to 1 above, characterized in that the adhesive porous layer has a porosity of 30% or more and 50% or less, and the polyvinylidene-fluoride-based resin contains vinylidene fluoride in an amount of 98 mol % or more. 3. The separator for a nonaqueous secondary battery according to 1 or 2 above, characterized in that the weight of the adhesive porous layer formed on one side of the porous substrate is 0.5 g/m² or more and 1.5 g/m² or less. 4. The separator for a nonaqueous secondary battery according to any one of 1 to 3 above, characterized in that the adhesive porous layer is formed on both front and back sides of the porous substrate. 5. The separator for a nonaqueous secondary battery according to 4 above, characterized in that the total weight of the adhesive porous layers formed on both sides of the porous substrate is 1.0 g/m² or more and 3.0 g/m² or less, and the difference between the weight of the adhesive porous layer on one side and the weight of the adhesive porous layer on the other side is 20% or less of the total weight. 6. The separator for a nonaqueous secondary battery according to any one of 1 to 5 above, characterized in that the porous substrate is a polyolefin microporous membrane containing polyethylene. 7. The separator for a nonaqueous secondary battery according to any one of 1 to 5 above, characterized in that the porous substrate is a polyolefin microporous membrane containing polyethylene and polypropylene. 8. The separator for a nonaqueous secondary battery according to 7 above, characterized in that the polyolefin microporous membrane includes at least two layers with one of the two layers containing polyethylene and the other layer containing polypropylene. 9. A nonaqueous secondary battery including the separator according to any one of 1 to 8 above.

Advantage of the Invention

According to the invention, it is possible to provide a separator for a nonaqueous secondary battery, which has better adhesion to electrodes compared with the prior art, is capable of ensuring sufficient ion permeability even after attachment to an electrode, and further includes an adhesive porous layer having dynamic physical properties sufficient to withstand heat pressing and a uniform porous structure. The use of the separator of the invention makes it possible to provide a high-energy-density, high-performance nonaqueous secondary battery having an aluminum laminate pack outer casing.

MODE FOR CARRYING OUT THE INVENTION

The separator for a nonaqueous secondary battery of the invention includes a porous substrate and an adhesive porous layer that is formed on at least one side of the porous substrate and contains a polyvinylidene-fluoride-based resin. The separator for a nonaqueous secondary battery is characterized in that the adhesive porous layer has a porosity of 30% or more and 60% or less and an average pore size of 1 nm or more and 100 nm or less. Hereinafter, the invention will be described in detail. Incidentally, a numerical range defined by “ . . . to . . . ” hereinafter indicates a numerical range including the upper limit and the lower limit.

[Porous Substrate]

In the invention, a porous substrate refers to a substrate having pores or voids inside. Examples of such substrates include a microporous membrane, a porous sheet made of a fibrous material, such as a nonwoven fabric or a paper-like sheet, and a composite porous sheet made of such a microporous membrane or porous sheet and one or more other porous layers stacked thereon. Incidentally, a microporous membrane refers to a membrane having a large number of micropores inside, the micropores being connected to allow gas or liquid to pass therethrough from one side to the other side.

The material forming the porous substrate may be an electrically insulating organic material or inorganic material. In particular, in terms of imparting a shutdown function to the substrate, it is preferable to use a thermoplastic resin as a component of the substrate. Here, a shutdown function refers to the following function: in the case where the battery temperature increases, the thermoplastic resin melts and blocks the pores of the porous substrate, thereby blocking the movement of ions to prevent the thermal runaway of the battery. As the thermoplastic resin, a thermoplastic resin having a melting point of less than 200° C. is suitable, and polyolefins are particularly preferable.

As a porous substrate using a polyolefin, a polyolefin microporous membrane is preferable. The polyolefin microporous membrane used may be a polyolefin microporous membrane that has been applied to a conventional nonaqueous secondary battery separator, which has sufficient dynamic physical properties and ion permeability. Further, in terms of having the shutdown function mentioned above, it is preferable that the polyolefin microporous membrane contains polyethylene, and it is preferable that the polyethylene content is 95 wt % or more.

Separately, in terms of imparting heat resistance to such a degree that the membrane does not easily break when exposed to high temperatures, a polyolefin microporous membrane containing polyethylene and polypropylene is preferable. An example of such a polyolefin microporous membrane is a microporous membrane in which both polyethylene and polypropylene are present in one sheet. In terms of achieving both the shutdown function and heat resistance, it is preferable that the microporous membrane contains polyethylene in an amount of 95 wt % or more and polypropylene in an amount of 5 wt % or less. In addition, in terms of achieving both the shutdown function and heat resistance, it is also preferable that the polyolefin microporous membrane is a polyolefin microporous membrane having a laminate structure, which includes at least two layers with one of the two layers containing polyethylene and the other layer containing polypropylene.

It is preferable that the polyolefin has a weight average molecular weight of 100,000 to 5,000,000. When the weight average molecular weight is less than 100,000, it may be difficult to ensure sufficient dynamic physical properties. Meanwhile, when it is more than 5,000,000, shutdown characteristics may deteriorate, or it may be difficult to form a membrane.

The polyolefin microporous membrane can be produced by the following method, for example. That is, it is possible to employ a method in which a microporous membrane is formed by successively performing the following steps: (i) a step of extruding a molten polyolefin resin from a T-die to form a sheet, (ii) a step of subjecting the sheet to a crystallization treatment, (iii) a step of stretching the sheet, and (iv) a step of heat-treating the sheet. In addition, it is also possible to employ a method in which a microporous membrane is formed by successively performing the following steps: (i) a step of melting a polyolefin resin together with a plasticizer such as liquid paraffin and extruding the melt from a T-die, followed by cooling to form a sheet, (ii) a step of stretching the sheet, (iii) a step of extracting the plasticizer from the sheet, and (iv) a step of heat-treating the sheet.

As a porous sheet made of a fibrous material, it is possible to use a porous sheet made of a fibrous material containing a polyester such as polyethylene terephthalate, a polyolefin such as polyethylene or polypropylene, or a heat-resistant polymer such as aromatic polyamide, polyimide, polyethersulfone, polysulfone, polyether ketone, or polyetherimide, or a mixture of such fibrous materials.

A composite porous sheet may be configured to include a microporous membrane or a porous sheet made of a fibrous material and a functional layer stacked thereon. Such a composite porous sheet is preferable because a further function can be imparted by the functional layer. As a functional layer, for example, in terms of imparting heat resistance, it is possible to use a porous layer made of a heat-resistant resin or a porous layer made of a heat-resistant resin and an inorganic filler. The heat-resistant resin may be one or more kinds of heat-resistant polymers selected from aromatic polyamide, polyimide, polyethersulfone, polysulfone, polyether ketone, and polyetherimide. Examples of suitable inorganic fillers include metal oxides such as alumina and metal hydroxides such as magnesium hydroxide. Incidentally, examples of techniques for forming a composite sheet include a method in which a porous sheet is coated with a functional layer, a method in which they are joined together using an adhesive, and a method in which they are bonded together by thermocompression.

In the invention, in terms of obtaining good dynamic physical properties and internal resistance, it is preferable that the porous substrate has a thickness within a range of 5 to 25 μm. In terms of preventing a short circuit in the battery and obtaining sufficient ion permeability, it is preferable that the porous substrate has a Gurley value (JIS P8117) within a range of 50 to 800 sec/100 cc. In terms of improving production yield, it is preferable that the porous substrate has a puncture resistance of 300 g or more.

[Polyvinylidene-Fluoride-Based Resin]

In the separator for a nonaqueous secondary battery of the invention, a polyvinylidene-fluoride-based resin is applied. The polyvinylidene-fluoride-based resin used may be a homopolymer of vinylidene fluoride (i.e., polyvinylidene fluoride), a copolymer of vinylidene fluoride and another copolymerizable monomer, or a mixture thereof. As a monomer copolymerizable with vinylidene fluoride, for example, it is possible to use one or more kinds of tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, trichloroethylene, vinyl fluoride, etc. Such a polyvinylidene-fluoride-based resin can be produced by emulsion polymerization or suspension polymerization.

It is preferable that the polyvinylidene-fluoride-based resin used in the separator for a nonaqueous secondary battery of the invention contains as a structural unit vinylidene fluoride in an amount of 98 mol % or more. In the case where vinylidene fluoride is contained in an amount of 98 mol % or more, more sufficient dynamic physical properties and heat resistance can be ensured even under severe heat-pressing conditions.

It is preferable that the polyvinylidene-fluoride-based resin for use in the invention has a weight average molecular weight within a range of 300,000 to 3,000,000. The weight average molecular weight is more preferably within a range of 300,000 to 2,000,000, and still more preferably within a range of 500,000 to 1,500,000. When the weight average molecular weight is less than 300,000, the adhesive porous layer may not have dynamic physical properties enough to withstand the step of attachment to an electrode, whereby sufficient adhesion may not be obtained. Meanwhile, when the weight average molecular weight is more than 3,000,000, a slurry containing the resin has high viscosity. As a result, it may be difficult to form an adhesive porous layer, or it may be impossible to form good crystals in the adhesive porous layer, making it difficult to obtain a preferred porous structure. Therefore, this is not preferable.

[Adhesive Porous Layer]

In the invention, the porous structure of the adhesive porous layer is an important technical factor. The porous structure is such that the porosity is 30 to 60% and the average pore size is 1 to 100 nm. Here, an adhesive porous layer refers to a porous layer containing a polyvinylidene-fluoride-based resin and having a large number of micropores inside, the micropores being connected to allow gas or liquid to pass therethrough from one side to the other side. The average pore size is calculated, assuming that all pores are cylindrical, from the following equation 1 using the pore surface area S of the adhesive porous layer calculated from the amount of nitrogen gas adsorption and the pore volume V of the adhesive porous layer calculated from porosity. d=4×V/S  (Equation 1)

d: Average pore size of adhesive porous layer

V: Pore volume of adhesive porous layer

S: Pore surface area of adhesive porous layer

In order to determine the pore surface area S of an adhesive porous layer, first, the specific surface area (m²/g) of a porous substrate and the specific surface area (m²/g) of a composite membrane including an adhesive porous layer applied in a nitrogen gas adsorption method are measured. Then, each specific surface area is multiplied by the respective weight per unit (g/m²) to determine the pore surface area per unit area of sheet, and the pore surface area of the porous substrate is subtracted from the pore surface area of the composite membrane to calculate the pore surface area S of the adhesive porous layer.

In the invention, the adhesive porous layer has a porosity of 30 to 60%. When the porosity of the adhesive porous layer is 30% or more, good ion permeability is obtained, and sufficient battery characteristics can be obtained. When the porosity of the adhesive porous layer is 60% or less, it is possible to obtain dynamic physical properties sufficient to prevent the porous structure from being destroyed during attachment to an electrode in the heat pressing process. In addition, when the porosity is 60% or less, the surface open-pore density is low, so a large area is occupied by the polyvinylidene-fluoride-based resin having an adhesive function, whereby sufficient adhesion strength can be ensured. Incidentally, it is more preferable that the adhesive porous layer has a porosity of 30 to 50%.

In the invention, the adhesive porous layer has an average pore size of 1 to 100 nm. When the average pore size of the adhesive porous layer is 100 nm or less, this is likely to lead to a porous structure in which uniform pores are uniformly dispersed, whereby points of attachment to an electrode are uniformly dispersed, making it possible to obtain good adhesion. In addition, in such a case, the movement of ions is also uniform. Accordingly, sufficient cycle characteristics can be obtained, and even better load characteristics can be obtained. Meanwhile, in terms of uniformity, it is preferable that the average pore size is as small as possible, but it is practically difficult to form a porous structure of less than 1 nm. In addition, although the polyvinylidene-fluoride-based resin swells in the case where the adhesive porous layer is impregnated with an electrolyte, when the average pore size is too small, the pores are blocked due to swelling, whereby ion permeability is inhibited. Also from such a point of view, it is preferable that the average pore size is 1 nm or more. Incidentally, it is more preferable that the adhesive porous layer has an average pore size of 20 to 100 nm.

Swelling with an electrolyte mentioned above varies depending on the kind of polyvinylidene-fluoride-based resin or the composition of the electrolyte, and the degree of swelling-related problems also varies. Focusing attention on the polyvinylidene-fluoride-based resin, for example, when a copolymer containing a copolymer component such as hexafluoropropylene in a large amount is used, the polyvinylidene-fluoride-based resin is prone to swelling. Therefore, in the invention, it is preferable to select a polyvinylidene-fluoride-based resin that does not cause the swelling-related problems mentioned above within the average pore size range of 1 to 100 nm. From such a point of view, it is preferable to use a polyvinylidene-fluoride-based resin containing vinylidene fluoride in an amount of 98 mol % or more.

In addition, focusing attention on the electrolyte, for example, when an electrolyte having a high content of a cyclic carbonate with high dielectric constant, such as ethylene carbonate or propylene carbonate, is used, the polyvinylidene-fluoride-based resin is prone to swelling. As a result, the swelling-related problems mentioned above are also likely to occur. In this respect, when a polyvinylidene-fluoride-based resin containing vinylidene fluoride in an amount of 98 mol % or more is used, even in the case where an electrolyte made only of a cyclic carbonate is used, sufficient ion permeability is obtained, and good battery performance is obtained. Therefore, this is preferable.

The adhesive porous layer in the invention is characterized in that although it has porosity suitable as a separator for a nonaqueous secondary battery, its average pore size is extremely smaller than the conventional adhesive porous layer. This means that the adhesive porous layer has a fine porous structure developed and the structure is uniform. According to such a porous structure, as mentioned above, the movement of ions at the interface between the separator and an electrode is made more uniform, whereby a uniform electrode reaction is obtained. Therefore, an improving effect on the load characteristics or cycle characteristics of a battery is obtained. In addition, the polyvinylidene-fluoride-based resin is uniformly distributed on the separator surface, resulting in even better adhesion to electrodes.

Further, the porous structure of the invention also improves the movement of ions at the interface between a porous substrate and the adhesive porous layer. Generally, in a laminate-type separator, clogging often occurs at the interface between two layers, reducing the movement of ions. Accordingly, it is sometimes difficult to obtain good battery characteristics. However, the adhesive porous layer in the invention has a fine porous structure developed, which is uniform and has a relatively large number of pores. Therefore, there is a high possibility that the pores of a porous substrate and the pores of the adhesive porous layer can be well connected, whereby clogging at the interface can also be significantly suppressed.

As mentioned above, in the separator of the invention, the adhesive porous layer has sufficient dynamic physical properties sufficient to withstand heat pressing and a uniform porous structure. Therefore, even in the case where the amount of binder resin in an electrode is reduced, and also the heat-pressing conditions are relaxed, better adhesion than the prior art is obtained, and sufficient ion permeability can be ensured even after attachment to an electrode. Therefore, the use of such a separator makes it possible to provide a high-energy-density, high-performance nonaqueous secondary battery having an aluminum laminate pack outer casing.

In order to obtain the adhesive porous layer as in the invention, a method that applies a polyvinylidene-fluoride-based resin having a high weight average molecular weight is mentioned, for example. Specifically, it is preferable to apply a polyvinylidene-fluoride-based resin having a weight average molecular weight of 600,000 or more, more preferably 1,000,000 or more. In addition, a method that increases the vinylidene fluoride content of a polyvinylidene-fluoride-based resin is also mentioned. Specifically, it is preferable to use a polyvinylidene-fluoride-based resin containing vinylidene fluoride in an amount of 98 mol % or more. In addition, a method in which the temperature of the coagulation bath is further reduced to form fine pores is also mentioned, for example.

Incidentally, it is also possible to incorporate a filler made of an inorganic substance or organic substance or other additives into the adhesive porous layer without inhibiting the effect of the invention. By incorporating such a filler, the slidability and heat resistance of the separator can be improved. Examples of usable inorganic fillers include metal oxides such as alumina and metal hydroxides such as magnesium hydroxide. Examples of usable organic fillers include acrylic resin.

[Separator for Nonaqueous Secondary Battery]

As mentioned above, the separator for a nonaqueous secondary battery of the invention includes a porous substrate and an adhesive porous layer that is formed on at least one side of the porous substrate and contains a polyvinylidene-fluoride-based resin. Here, the adhesive porous layer is an adhesive layer to be attached, with an electrolyte contained therein, to an electrode by heat pressing. Therefore, it is necessary that the adhesive porous layer is present as the outermost layer of the separator. Naturally, in terms of cycle life, it is preferable that the separator is attached to both the positive electrode and the negative electrode. Therefore, it is preferable that the adhesive porous layer is formed on the front and back of the porous substrate.

In the invention, in terms of ion permeability, it is preferable that the adhesive porous layer has a structure that is sufficiently porous. Specifically, it is preferable that the difference between the Gurley value of the porous substrate used and the Gurley value of the composite separator after forming an adhesive porous layer is 300 sec/100 cc or less, more preferably 150 sec/100 cc or less, and still more preferably 100 sec/100 cc or less. In the case where the difference is more than 300 sec/100 cc, the adhesive porous layer may be so dense that ion permeation is inhibited, whereby sometimes sufficient battery characteristics cannot be obtained.

In terms of obtaining sufficient battery performance, it is preferable that the separator for a nonaqueous secondary battery of the invention has a Gurley value within a range of 50 sec/100 cc or more and 800 sec/100 cc or less.

In order for the effect of the invention and the dynamic physical properties of the separator to be well obtained, a suitable range of the porosity of the separator for a nonaqueous secondary battery is 30% or more and 60% or less.

In terms of adhesion to electrodes, ion permeability, and the load characteristics of a battery, it is preferable that the weight of the polyvinylidene-fluoride-based resin is within a range of 0.5 g/m² or more and 1.5 g/m² or less on one side. In the case where the adhesive porous layer is formed on both front and back sides, it is preferable that the total weight of the polyvinylidene-fluoride-based resin is 1.0 g/m² or more and 3.0 g/m² or less.

In the invention, in the case where the adhesive porous layer is formed on both sides of the porous substrate, the difference between the weight on the front side and the weight on the back side is also important. Specifically, it is preferable that the total weight of the adhesive porous layers formed on the front and back of the porous substrate is 1.0 to 3.0 g/m², and the difference between the weight of the adhesive porous layer on one side and the weight of the adhesive porous layer on the other side is 20% or less of the total weight. When the difference is more than 20%, this may result in significant curling, interfering with handling or deteriorating cycle characteristics.

In the invention, in terms of ensuring good ion permeability, it is preferable that the tortuosity of the separator for a nonaqueous secondary battery is within a range of 1.5 to 2.5. In terms of mechanical strength and energy density, it is preferable that the membrane thickness of the separator for a nonaqueous secondary battery is 5 to 35 μm. In terms of ensuring adhesion and good ion permeability, it is preferable that the membrane thickness of the adhesive porous layer on one side is within a range of 0.5 to 5 μm. In terms of cycle characteristics, it is preferable that the fibril diameter of the polyvinylidene-fluoride-based resin in the adhesive porous layer is within a range of 10 to 1,000 nm.

In terms of ensuring the sufficient load characteristics of a battery, it is preferable that the membrane resistance of the separator for a nonaqueous secondary battery is within a range of 1 to 10 ohm·cm². Membrane resistance herein is the resistance of a separator impregnated with an electrolyte and is measured by an AC method. The value naturally varies depending on the kind of electrolyte or the temperature, and the above value is a value measured at 20° C. using 1 M LiBF₄ propylene carbonate/ethylene carbonate (weight ratio: 1/1) as an electrolyte.

[Method for Producing Separator for Nonaqueous Secondary Battery]

The separator for a nonaqueous secondary battery of the invention mentioned above can be produced by a method in which a solution containing a polyvinylidene-fluoride-based resin is applied directly onto a porous substrate, and then the polyvinylidene-fluoride-based resin is solidified to integrally form an adhesive porous layer on the porous substrate.

Specifically, first, a polyvinylidene-fluoride-based resin is dissolved in a solvent to prepare a coating liquid. The coating liquid is applied onto a porous substrate, followed by immersion in a suitable coagulation liquid. As a result, the polyvinylidene-fluoride-based resin is solidified while inducing a phase separation phenomenon. In this step, the polyvinylidene-fluoride-based resin layer has a porous structure. Subsequently, the porous substrate is washed with water to remove the coagulation liquid, followed by drying to integrally form an adhesive porous layer on the porous substrate.

For the coating liquid, a good solvent that dissolves the polyvinylidene-fluoride-based resin can be used. Examples of suitable good solvents include polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and dimethylformamide. In terms of forming a good porous structure, in addition to the good solvent, a phase-separation agent that induces phase separation is preferably mixed. Examples of phase-separation agents include water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol. It is preferable that such a phase-separation agent is added within a range where viscosity suitable for coating can be ensured. In addition, in the case where a filler or other additives are incorporated into the adhesive porous layer, they may be mixed or dissolved into the coating liquid.

The composition of the coating liquid is preferably such that the polyvinylidene-fluoride-based resin is contained at a concentration of 3 to 10 wt %. In terms of forming a suitable porous structure, it is preferable that the solvent is a mixed solvent containing a good solvent in an amount of 60 wt % or more and a phase-separation agent in an amount of 40 wt % or less.

As the coagulation liquid, it is possible to use water, a mixed solvent of water and the good solvent, or a mixed solvent of water, the good solvent, and the phase-separation agent. In particular, a mixed solvent of water, the good solvent, and the phase-separation agent is preferable. In such a case, in terms of productivity, it is preferable that the mixing ratio between the good solvent and the phase-separation agent is determined according to the mixing ratio of the mixed solvent used for dissolving the polyvinylidene-fluoride-based resin. In terms of forming a good porous structure and improving productivity, it is preferable that the concentration of water is 40 to 90 wt %.

For the application of the coating liquid to a porous substrate, a conventional coating technique can be employed, such as Mayer bar, die coater, reverse roll coater, or gravure coater. In the case where the adhesive porous layer is formed on both sides of the porous substrate, it is possible that the coating liquid is applied to one side and then to the other, coagulated, washed with water, and dried. However, in terms of productivity, it is preferable that the coating liquid is applied to both sides of the porous substrate simultaneously, coagulated, washed with water, and dried.

Incidentally, in addition to the wet coating method mentioned above, the separator of the invention can also be produced by a dry coating method. Here, a dry coating method is a method in which the coating liquid containing a polyvinylidene-fluoride-based resin and a solvent is applied onto a porous substrate and then dried to remove the solvent by volatilization, thereby obtaining a porous membrane. However, in the case of a dry coating method, as compared with a wet coating method, it is more likely that the resulting coating is a dense membrane, and it is almost impossible to obtain a porous layer unless a filler or the like is added to the coating liquid. In addition, even when such a filler or the like is added, it is difficult to obtain a good porous structure. Therefore, from such a point of view, it is preferable to use a wet coating method in the invention.

In addition, the separator of the invention can also be produced by a method in which an adhesive porous layer and a porous substrate are separately produced, and then these sheets are stacked together and combined by thermocompression bonding or using an adhesive, etc. The method for obtaining an adhesive porous layer as an independent sheet may be, for example, a method in which a coating liquid is applied onto a release sheet, then an adhesive porous layer is formed by the wet coating method or dry coating method mentioned above, and only the adhesive porous layer is peeled off.

[Nonaqueous Secondary Battery]

The nonaqueous secondary battery of the invention is characterized by using the separator of the invention mentioned above.

In the invention, a nonaqueous secondary battery has a configuration in which the separator is placed between a positive electrode and a negative electrode, and these battery elements are enclosed in an outer casing together with an electrolyte. As the nonaqueous secondary battery, a lithium ion secondary battery is preferable.

As the positive electrode, a configuration in which an electrode layer made of a positive electrode active material, a binder resin, and a conductive additive is formed on a positive electrode current collector can be applied. Examples of positive electrode active materials include lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide having a spinel structure, and lithium iron phosphate having an olivine structure. In the invention, in the case where the adhesive porous layer of the separator is placed on the positive-electrode side, because of the excellent oxidation resistance of the polyvinylidene-fluoride-based resin, there also is an advantage in that a positive electrode active material that can be operated at a high voltage of 4.2 V or more, such as LiMn_(1/2)Ni_(1/2)O₂ or LiCo_(1/3)Mn_(1/3)Ni_(1/3)O₂, can be easily applied. Examples of binder resins include polyvinylidene-fluoride-based resins. Examples of conductive additives include acetylene black, ketjen black, and graphite powder. Examples of current collectors include an aluminum foil having a thickness of 5 to 20 μm.

As the negative electrode, a configuration in which an electrode layer made of a negative electrode active material and a binder resin is formed on a negative electrode current collector can be applied. A conductive additive may also be added to the electrode layer as necessary. Examples of usable negative electrode active materials are carbon materials capable of electrochemically storing lithium, materials capable of alloying with lithium, such as silicon and tin, and the like. Examples of binder resins include polyvinylidene-fluoride-based resins and styrene-butadiene rubber. The separator for a nonaqueous secondary battery of the invention has good adhesion. Therefore, sufficient adhesion can be ensured not only when a polyvinylidene-fluoride-based resin is used as a negative electrode binder, but also in the case of using styrene-butadiene rubber. In addition, examples of conductive additives include acetylene black, ketjen black, and graphite powder. Examples of current collectors include a copper foil having a thickness of 5 to 20 μm. In addition, instead of the negative electrode mentioned above, a metal lithium foil may also be used as a negative electrode.

The electrolyte has a configuration in which a lithium salt is dissolved in a suitable solvent. Examples of lithium salts include LiPF₆, LiBF₄, and LiClO₄. Examples of suitable solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, fluoroethylene carbonate, and difluoroethylene carbonate, open-chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluorine substitution products thereof, cyclic esters such as γ-butyrolactone and γ-valerolactone, and mixed solvents thereof. In particular, a solvent obtained by dissolving a lithium salt in a solvent containing cyclic carbonate/open-chain carbonate in a weight ratio of 20 to 40/80 to 60 at a concentration of 0.5 to 1.5 M is preferable. Incidentally, in a separator including a conventional adhesive porous layer, depending on the kind of electrolyte used, adhesion to electrodes is sometimes hardly exhibited. However, in the separator of the invention, good adhesion can be exhibited regardless of the kind of electrolyte. The invention is also greatly advantageous in this respect.

The separator for a nonaqueous secondary battery of the invention is applicable to a battery having a metal can outer casing. However, because of its good adhesion to electrodes, the separator of the invention is suitable for use in a soft pack battery having an aluminum laminate film outer casing. The method for producing such a battery is as follows. The positive electrode and negative electrode mentioned above are joined together via the separator, impregnated with an electrolyte, enclosed in an aluminum laminate film, and then heat-pressed, whereby a nonaqueous secondary battery can be obtained. This configuration of the invention allows the electrodes and the separator to be well attached together, making it possible to obtain a nonaqueous secondary battery having excellent cycle life. In addition, because of the good adhesion between the electrodes and the separator, the battery also has excellent safety. Examples of methods for joining the electrodes and the separator together include a stacking method in which the electrodes and the separator are stacked together and a method in which the electrodes and the separator are wound together. The invention is applicable to any of such methods.

EXAMPLES

Hereinafter, the invention will be described with reference to examples. However, the invention is not limited to the following examples.

[Measurement Method]

(Average Pore Size of Adhesive Porous Layer)

By a gas adsorption method, applying a BET method, the specific surface area (m²/g) of a polyolefin microporous membrane and the specific surface area (m²/g) of a composite separator including an adhesive porous layer are measured. These specific surface areas (m²/g) are multiplied by respective weights per unit (g/m²) to calculate the pore surface areas per m² of sheet. The pore surface area of the polyolefin microporous membrane is then subtracted from the pore surface area of the composite separator to calculate the pore surface area S per m² of adhesive porous layer. Separately, the pore volume V per m² of sheet is calculated from porosity. Here, assuming that all pores are cylindrical, the average pore size (diameter) d is calculated from the following equation 2 using the pore surface area S and the pore volume V. d=4×V/S  (Equation 2)

d: Average pore size of adhesive porous layer

V: Pore volume of adhesive porous layer

S: Pore surface area of adhesive porous layer

The obtained d was defined as the average pore size of the adhesive porous layer made of a polyvinylidene-fluoride-based-resin.

(Membrane Thickness)

Measurement was performed using a contact thickness meter (LITEMATIC, manufactured by Mitutoyo). Using a cylindrical measuring terminal having a diameter of 5 mm, the thickness was measured with adjustment so that a load of 7 g was applied during the measurement.

(Weight Per Unit)

A 10 cm×10 cm sample was cut out and measured for weight. The weight was divided by the area to determine the weight per unit.

(Weight of Polyvinylidene-Fluoride-Based Resin)

Using an energy dispersion fluorescent X-ray analyzer (EDX-800HS, Shimadzu), the weight of a polyvinylidene-fluoride-based resin was measured from the intensity of the FKα spectrum. In the measurement, the weight of the polyvinylidene-fluoride-based resin on the X-ray irradiated side is measured. Therefore, in the case where a porous layer made of a polyvinylidene-fluoride-based resin is formed on both front and back sides, the front and back are each subjected to the measurement. Thus, the weight of the polyvinylidene-fluoride-based resin on each of the front and back sides is measured, and the weights are summed to determine the total weight on the front and back sides.

(Porosity)

The porosity ∈ (%) of a complex separator was calculated from the following equation 3. ∈={1−(Wa/0.95+Wb/1.78)/t}×100  (Equation 3)

Here, Wa is the weight per unit of a substrate (g/m²), Wb is the weight of a polyvinylidene-fluoride-based resin (g/m²), and t is the membrane thickness (μm).

In the case where the porosity of an adhesive porous layer is to be calculated, it can be determined from the above equation 3, in which Wa=0 (g/m²), and t is the thickness of the adhesive porous layer, i.e., the value obtained by subtracting the membrane thickness of a substrate from the membrane thickness of a composite separator.

(Gurley Value)

Measurement was performed in accordance with JIS P8117 using a Gurley densometer (G-B2C, manufactured by Toyo Seiki Seisaku-Sho).

Example 1

A KF polymer #9300 manufactured by Kureha Kagaku Kogyo (vinylidene fluoride/hexafluoropropylene=98.9/1.1 mol %, weight average molecular weight: 1,950,000) was used as a polyvinylidene-fluoride-based resin. The polyvinylidene-fluoride-based resin was dissolved in a mixed solvent containing dimethylacetamide/tripropylene glycol in a weight ratio of 7/3 at a concentration of 5 wt % to prepare a coating liquid. Equal amounts of the coating liquid were applied respectively to both sides of a polyethylene microporous membrane (TN0901, manufactured by SK) having a membrane thickness of 9 μm, a Gurley value of 160 sec/100 cc, and a porosity of 43%, followed by immersion in a coagulation liquid (40° C.) containing water/dimethylacetamide/tripropylene glycol in a weight ratio of 57/30/13 to cause solidification. The microporous membrane was then washed with water, followed by drying to give a separator for a nonaqueous secondary battery according to the invention having an adhesive porous layer made of a polyvinylidene-fluoride-based resin on both front and back sides of the polyolefin-based microporous membrane. The separator was measured for the following properties: the membrane thickness of the separator, the VDF content of the polyvinylidene-fluoride-based resin, the porosity of the separator (whole) and that of the adhesive porous layer (PVdF layer), the average pore size and weight of the adhesive porous layer (PVdF layer) (the total weight on both sides, the weight on the front side, the weight on the back side, and the percentage of the difference between the weight on the front side and the weight on the back side relative to the total weight on both sides), and the Gurley value of the separator. The results are shown in Table 1. Incidentally, the results from the separators of the following examples and comparative examples are also shown in Table 1.

Examples 2 to 5

Separators for a nonaqueous secondary battery according to the invention were obtained using the same coating liquid, the same polyethylene microporous membrane, and the same method as in Example 1, except that only the amount applied was changed as shown in Table 1.

Examples 6 and 7

Separators for a nonaqueous secondary battery according to the invention were obtained using the same coating liquid, the same polyethylene microporous membrane, and the same method as in Example 1, except that only the amounts applied to the front and back were changed as shown in Table 1.

Example 8

A separator for a nonaqueous secondary battery according to the invention was obtained in the same manner as in Example 1, except that a polyolef in microporous membrane (P4824, Celgard) having a three-layer structure including polypropylene/polyethylene/polypropylene with a membrane thickness of 12 μm, a Gurley value of 425 sec/100 cc, and a porosity of 38% was used as a polyolefin microporous membrane.

Example 9

A polyvinylidene-fluoride-based resin having a copolymerization composition of vinylidene fluoride/hexafluoropropylene/chlorotrifluoroethylene in a weight ratio of 92.0/4.5/3.5 was produced by emulsion polymerization. The weight average molecular weight of the polyvinylidene-fluoride-based resin was 410,000. The polyvinylidene fluoride was used such that the amount applied to both sides was 0.8 g/m². Otherwise in the same manner as in Example 1, a separator for a nonaqueous secondary battery according to the invention was obtained.

Comparative Example 1

A polyvinylidene-fluoride-based resin having a copolymerization composition of vinylidene fluoride/hexafluoropropylene/chlorotrifluoroethylene in a weight ratio of 92.0/4.5/3.5 was produced by emulsion polymerization. The weight average molecular weight of the polyvinylidene-fluoride-based resin was 410,000. The polyvinylidene fluoride was dissolved in a mixed solvent containing dimethylacetamide/tripropylene glycol in a weight ratio of 60/40 at a concentration of 12 wt % to prepare a coating liquid. Equal amounts of the coating liquid were applied respectively to both sides of a polyethylene microporous membrane (TN0901, manufactured by SK) having a membrane thickness of 9 μm, a Gurley value of 160 sec/100 cc, and a porosity of 43%, followed by immersion in a coagulation liquid (40° C.) containing water/dimethylacetamide/tripropylene glycol in a weight ratio of 50/30/20 to cause solidification. The microporous membrane was then washed with water, followed by drying to give a separator for a nonaqueous secondary battery having a porous layer made of a polyvinylidene-fluoride-based resin on the polyolefin-based microporous membrane.

Comparative Example 2

A separator for a nonaqueous secondary battery was obtained in the same manner as in Comparative Example 1, except that the coating liquid composition was such that the polyvinylidene-fluoride-based resin concentration was 8 wt % and the weight ratio of dimethylacetamide/tripropylene glycol was 55/45.

Comparative Example 3

A separator for a nonaqueous secondary battery was obtained in the same manner as in Example 1, except that the temperature of the coagulation liquid was 0° C.

Comparative Example 4

A KF polymer #9300 manufactured by Kureha Kagaku Kogyo (vinylidene fluoride/hexafluoropropylene=98.9/1.1 mol %, weight average molecular weight: 1,950,000) was used as a polyvinylidene-fluoride-based resin. The polyvinylidene fluoride was dissolved in NMP at 5 wt % to prepare a coating liquid. Equal amounts of the coating liquid were applied respectively to both sides of a polyethylene microporous membrane (TN0901, manufactured by SK) having a membrane thickness of 9 μm, a Gurley value of 160 sec/100 cc, and a porosity of 43%, followed by vacuum drying at 100° C. for 12 hours. However, the obtained polyvinylidene fluoride membrane was dense, and it was not possible to form an adhesive porous layer.

[Measurement of Resistance of Separator Impregnated with Electrolyte]

1M LiBF₄ propylene carbonate/ethylene carbonate (weight ratio: 1/1) was used as an electrolyte. A separator was impregnated with the electrolyte, then sandwiched between aluminum foil electrodes each provided with a lead tab, and enclosed in an aluminum pack to produce a test cell. The resistance of the test cell was measured at 20° C. and −20° C. by an AC impedance method (measurement frequency: 100 kHz). The measurement was performed for Example 1, Comparative Examples 1 to 3, and the above polyethylene microporous membrane (TN0901, manufactured by SK). The results are shown in Table 2. In addition, from the obtained resistance at 20° C., tortuosity was calculated using the following equation 4. The results are also shown in Table 2. τ={(R*∈/100)/(r*t)}^(1/2)  (Equation 4)

τ: Tortuosity

R (ohm·cm²): Resistance of separator impregnated with electrolyte

r (ohm·cm): Specific resistance of electrolyte

∈ (%): Porosity

t (cm): Membrane thickness

[Interpretation of Results of Resistance Measurement]

With respect to values obtained by dividing the resistance at −20° C. by the resistance at 20° C., the values in Example 1, Comparative Example 2, and the polyolefin microporous membrane are comparable, while the value in Comparative Example 1 is significantly greater. When an adhesive porous layer is impregnated with an electrolyte, the rate of movement of electrolyte ions present in the swollen resin is extremely slower as compared with electrolyte ions present in the pores, and the difference is more significant at a lower temperature. This leads to the presumption that in Comparative Example 1, the amount of electrolyte contained in the swollen resin is greater and the amount of electrolyte independently present in the pores is smaller than in Example 1, and, therefore, there was a difference in the movement of ions, resulting in the higher resistance at a low temperature. The polyvinylidene-fluoride-based resin applied in Comparative Examples 1 and 2 is more prone to swelling with an electrolyte as compared with Example 1. In the case where such a resin is applied, a decrease in pore size increases the amount of ions in the swollen resin. Therefore, in the case where such a resin is applied, a decrease in the pore size of a porous layer made of the polyvinylidene-fluoride-based resin decreases ion permeability, particularly ion permeability at a low temperature, and thus is undesirable. However, in Example 1, although the pore size is small, an increase in resistance at a low temperature is small. This is because the polyvinylidene-fluoride-based resin in Example 1 is relatively resistant to swelling with an electrolyte. This shows that it is desirable to select a polyvinylidene-fluoride-based resin suitable for the porous structure of an adhesive porous layer.

Tortuosity comparison between Example 1 and Comparative Examples 1 to 3 shows that tortuosity of Example 1 is significantly lower. Considering the fact that the same polyolefin microporous membrane is used in all samples and that the adhesive porous layer thickness is significantly smaller than the polyolefin microporous membrane thickness, it is presumed that tortuosity reflects clogging at the interface between the polyolefin microporous membrane and the adhesive porous layer. That is, it can be said that higher tortuosity indicates a greater degree of clogging. As mentioned above, in the systems of comparative examples, a polyvinylidene-fluoride-based resin that is prone to swelling with an electrolyte is applied. Therefore, clogging due to swelling also has to be considered. In light of the above descriptions, the effect of swelling is expected to be small in the system of Comparative Example 2 where the average pore size is large. However, tortuosity is significantly high even in such a system. This is likely to reflect that because the adhesive porous layer in the invention is dense and uniformly porous, the probability of clogging is low. Accordingly, it is expected that the porous structure of the invention makes it possible to prevent clogging at the interface, and the configuration of the invention is thus preferable in terms of the movement of ions.

In Comparative Example 3, the porosity of the porous layer made of a polyvinylidene-fluoride-based resin is low. In such a configuration, the number of pores formed in the porous layer made of a polyvinylidene-fluoride-based resin is small, and, therefore, an interface with a polyolefin microporous membrane is not formed well. Accordingly, clogging is significant, and tortuosity is thus high. Thus, the membrane resistance is high both at 20° C. and −20° C., and sufficient ion permeability is not obtained. Incidentally, this tendency is more pronounced at −20° C.

[Production of Nonaqueous Secondary Battery]

(Production of Negative Electrode)

300 g of artificial graphite (MCMB25-28, manufactured by Osaka Gas Chemicals) as a negative electrode active material, 7.5 g of “BM-400B” manufactured by Zeon (a water-soluble dispersion containing a modification product of a styrene-butadiene copolymer in an amount of 40 wt %) as a binder, 3 g of carboxymethylcellulose as a thickener, and an appropriate amount of water were stirred in a double-arm mixer to prepare a slurry for a negative electrode. The slurry for a negative electrode was applied to a copper foil having a thickness of 10 μm as a negative electrode current collector, and the resulting coating was dried, followed by pressing to prepare a negative electrode having a negative electrode active material layer.

(Production of Positive Electrode)

89.5 g of a powder of lithium cobalt oxide (CELLSEED C, manufactured by Nippon Chemical Industrial) as a positive electrode active material, 4.5 g of acetylene black (DENKA BLACK, manufactured by Denki Kagaku Kogyo) as a conductive additive, and 6.0 g of polyvinylidene fluoride (KF polymer W#1100, manufactured by Kureha Kagaku Kogyo) as a binder were dissolved in NMP, and the resulting solution was stirred in a double-arm mixer in such a manner that the polyvinylidene fluoride was in an amount of 6 wt %, thereby preparing a slurry for a positive electrode. The slurry for a positive electrode was applied to an aluminum foil having a thickness of 20 μm as a positive electrode current collector, and the resulting coating was dried, followed by pressing to prepare a positive electrode having a positive electrode active material layer.

(Production of Battery)

A lead tab was welded to the positive electrode and the negative electrode. The positive and negative electrodes were then joined together via a separator, impregnated with an electrolyte, and enclosed in an aluminum pack using a vacuum sealer. Here, 1 M LiPF₆ ethylene carbonate/ethyl methyl carbonate (weight ratio: 3/7) was used as the electrolyte. The aluminum pack was then heat-pressed in a heat press at 90° C. for 2 minutes while applying a load of 20 kg per cm² of electrode, thereby producing a test battery.

[Load Characteristic Test]

The load characteristic test was performed using nonaqueous secondary batteries produced as above. The relative discharge capacity at 2 C with respect to the discharge capacity at 0.2 C was measured at 25° C. and used as an index of the load characteristics of a battery. The test was performed on batteries using the separators of Examples 1 to 9 and Comparative Examples 1 to 4, respectively. The results are shown in Table 3.

[Charge-Discharge Cycle Test]

The charge-discharge cycle test was performed using nonaqueous secondary batteries produced as above. The charge condition was constant-current constant-voltage charge at 1 C and 4.2 V, while the discharge condition was constant-current discharge at 1 C and 2.75 V cut-off, and the cycle characteristics were thus tested. Here, capacity retention after 100 cycles was used as an index of cycle characteristics. The test was performed on batteries using the separators of Examples 1 to 9 and Comparative Examples 1 to 4, respectively. The results are shown in Table 3.

[Check for Adhesion to Electrodes]

The batteries after the charge-discharge cycle test were each disassembled and checked for the adhesion between the separator and the electrodes. The adhesion was examined in terms of adhesion strength and uniformity. The results are shown in Table 3. Incidentally, in Table 3, adhesion strength on each of the positive-electrode side and the negative-electrode side is shown as a relative value taking the peel strength in the case of using the separator of Example 1 as 100. With respect to uniformity, after a peel test was performed on each of the positive-electrode side and the negative-electrode side, when almost the entire adhesive porous layer remained attached to the electrode surface, uniformity was rated as good (A); when most of the adhesive porous layer remained attached to the electrode surface, but the layer was partially broken, uniformity was rated as fair (B); and when most of the adhesive porous layer did not remain attached to the electrode surface, and the layer was significantly broken, uniformity was rated as poor (C).

In Comparative Examples 1 and 2, adhesion is poor. This is because the adhesive porous layer had large average pore size and high porosity, and thus was not able to dynamically withstand heat pressing, which is the step of attachment to an electrode; as a result, the adhesive porous layer was not maintained between the electrode and the separator and ran out. In contrast, this did not happen in Examples 1 to 9. From such a point of view, it is shown that the average pore size and porosity of the separator for a nonaqueous secondary battery of the invention are preferable.

[Heat Resistance Evaluation]

The heat resistance of the separator of Example 1 and that of the separator of Example 8 were compared by thermal mechanical analysis (TMA). Specifically, each separator was cut to a width of 4 mm, and both ends thereof were held in chucks and set to a chuck-to-chuck distance of 10 mm. Under an applied load of 10 mN, the temperature was raised at a temperature rise rate of 10° C./min, and the temperature at which the separator broke was measured. It was found that the separator of Example 1 broke at 155° C., while the separator of Example 8 broke at 180° C. This shows that the use of polypropylene is preferable in terms of heat resistance.

[Kind of Electrolyte and Adhesion]

The separators of Example 1 and Comparative Examples 1 to 4 were tested for adhesion to electrodes in the same manner as above using various electrolytes. Incidentally, 1 M LiPF₆ ethylene carbonate/ethyl methyl carbonate (weight ratio: 3/7) was used as an electrolyte A, 1 M LiPF₆ ethylene carbonate/propylene carbonate/ethyl methyl carbonate (weight ratio: 3/2/5) was used as an electrolyte B, and 1 M LiPF₆ ethylene carbonate/propylene carbonate (weight ratio: 1/1) was used as an electrolyte C. Table 4 shows the results. Incidentally, in Table 4, in terms of the average of the peel strengths on the positive electrode and the negative electrode expressed as relative values with respect to the peel strengths of the separator of Example 1 obtained on the positive electrode and the negative electrode, respectively, each taken as 100, an average of 70 or more is shown as Good, an average of 50 or more and less than 70 is shown as Fair, an average of less than 50 is shown as Poor.

TABLE 1 Average Weight of PVdF (g/m²) Membrane VDF Porosity (%) Pore Size of Difference Gurley Thickness Content PVdF PVdF Layer between Front Value (μm) (mol %) Whole Layer (nm) Total Front Back and Back (sec/100 cc) Example 1 11 98.9 42 38 44 2.2 1.1 1.1  0% 241 Example 2 10 98.9 43 36 51 0.9 0.5 0.4 11% 225 Example 3 10 98.9 42 34 48 1.6 0.8 0.8  0% 237 Example 4 11 98.9 41 33 35 2.6 1.3 1.3  0% 261 Example 5 13 98.9 47 55 68 3.4 1.7 1.7  0% 364 Example 6 11 98.9 42 38 45 2.2 1.3 0.9 18% 240 Example 7 11 98.9 42 39 41 2.2 1.6 0.6 45% 243 Example 8 14 98.9 42 38 44 2.2 1.1 1.1  0% 499 Example 9 10 92 44 50 95 0.8 0.4 0.4  0% 240 Comparative 12 92 47 59 115 2.2 1.1 1.1  0% 324 Example 1 Comparative 13 92 50 66 292 2.6 1.3 1.3  0% 248 Example 2 Comparative 11 98.9 40 25 10 2.2 1.1 1.1  0% 545 Example 3 Comparative 10 98.9 38 0 No pores 2.2 1.1 1.1  0% 999 or more Example 4

TABLE 2 Results of Resistance Measurement 20° C. Tortuosity −20° C. (ohm · cm²) — (ohm · cm²) −20° C./20° C. Example 1 3.17 2.03 12.7 4.0 Comparative 4.08 2.25 58.7 14.4 Example 1 Comparative 4.10 2.30 16.6 4.0 Example 2 Comparative 6.45 2.84 34.2 5.3 Example 3 Polyolefin 2.46 1.98 10.1 4.1 Microporous Membrane

TABLE 3 Check for Adhesion to Electrodes Results of Positive Electrode Negative Electrode Results of Load Charge-Discharge Adhesion Adhesion Characteristic Test Cycle Test Strength Uniformity Strength Uniformity Example 1 94% 96% 100 A 100 A Example 2 94% 91% 90 A 85 A Example 3 94% 95% 95 A 94 A Example 4 93% 96% 102 A 110 A Example 5 90% 96% 103 A 115 A Example 6 94% 90% 102 A 86 A Example 7 93% 79% 103 B 80 B Example 8 89% 75% 101 A 103 A Example 9 94% 90% 89 B 79 B Comparative 70% 43% 10 C 5 C Example 1 Comparative 65% 40% 8 C 1 C Example 2 Comparative 15% 10% 110 A 120 A Example 3 Comparative 11%  7% 105 A 113 A Example 4

TABLE 4 Adhesion Electrolyte A Electrolyte B Electrolyte C Example 1 Good Good Good Comparative Example 1 Poor Fair Good Comparative Example 2 Poor Fair Good Comparative Example 3 Good Good Good Comparative Example 4 Good Good Good

INDUSTRIAL APPLICABILITY

The nonaqueous secondary battery separator of the invention is suitable for use in a nonaqueous secondary battery. The separator is particularly suitable for use in a nonaqueous secondary battery having an aluminum laminate outer casing, where joining to electrodes is important. 

The invention claimed is:
 1. A separator for a nonaqueous secondary battery, comprising a porous substrate and an adhesive porous layer that is formed on at least one side of the porous substrate and contains a polyvinylidene-fluoride-based resin, the separator for a nonaqueous secondary battery being characterized in that the adhesive porous layer has a porosity of 30% or more and 60% or less and an average pore size of 1 nm or more and 100 nm or less, the polyvinylidene-fluoride-based resin having a weight average molecular weight of 600,000 or more.
 2. The separator for a nonaqueous secondary battery according claim 1, characterized in that the adhesive porous layer has a porosity of 30% or more and 50% or less, and the polyvinylidene-fluoride-based resin contains vinylidene fluoride in an amount of 98 mol % or more.
 3. The separator for a nonaqueous secondary battery according to claim 1, characterized in that a weight of the adhesive porous layer formed on one side of the porous substrate is 0.5 g/m² or more and 1.5 g/m² or less.
 4. The separator for a nonaqueous secondary battery according to claim 1, characterized in that the adhesive porous layer is formed on both front and back sides of the porous substrate.
 5. The separator for a nonaqueous secondary battery according to claim 4, characterized in that a total weight of the adhesive porous layers formed on both sides of the porous substrate is 1.0 g/m² or more and 3.0 g/m² or less, and the difference between the weight of the adhesive porous layer on one side and the weight of the adhesive porous layer on the other side is 20% or less of the total weight.
 6. The separator for a nonaqueous secondary battery according to claim 1, characterized in that the porous substrate is a polyolefin microporous membrane containing polyethylene.
 7. The separator for a nonaqueous secondary battery according to claim 1, characterized in that the porous substrate is a polyolefin microporous membrane containing polyethylene and polypropylene.
 8. The separator for a nonaqueous secondary battery according to claim 7, characterized in that the polyolefin microporous membrane includes at least two layers with one of the two layers containing polyethylene and the other layer containing polypropylene.
 9. A nonaqueous secondary battery comprising the separator according to claim
 1. 10. The separator for a nonaqueous secondary battery according to claim 2, characterized in that a weight of the adhesive porous layer formed on one side of the porous substrate is 0.5 g/m² or more and 1.5 g/m² or less. 